(1) Field of the Invention
The present invention relates to an SDH (Synchronous Digital Hierarchy) transmission system, and a frame transmission method in an SDH transmission system and an SDH transmission unit, and more particularly to a transmission method suitable for use in a SONET in accordance with an SDH transmission mode.
(2) Description of the Related Art
(A) Description of Transmission Frame Handled in SONET (SDH) Transmission Mode
FIG. 9 is a diagram showing a format of a basic transmission frame handled in a SONET (Synchronous Optical Network). As shown in FIG. 9, the basic transmission frame for the SONET has 9xc3x973 bytes of overhead 10 containing various operation and maintenance (supervisory control) information such as frame synchronization signal, or parity check signal, and 9xc3x9787 bytes of payload 20 containing actually transmitted data, resulting in information of 9xc3x9790 bytes in total. In the SONET, the 90xc3x979 bytes (=810 bytes) of frame is transmitted 8000 times per second, thereby generating a signal [referred to as STS-1 (Synchronous Transport Signal Level 1) , or STM-0 (Synchronous Transfer Module Level 0) in the SDH] having a transmission rate of 51.84M (=90xc3x979xc3x978xc3x978000) b/s. As used herein, xe2x80x9cSONETxe2x80x9d means a network currently used in North America in accordance with an SDH transmission mode.
Further, as is well known, the overhead 10 is provided with a section overhead [(R-) SOH] 11 which, in communication between a line terminal multiplex relay transmission unit (LTE) and a regenerator unit (REG), or between the regenerator units (REG) (referred to as xe2x80x9csectionxe2x80x9d in the SONET, or xe2x80x9cregenerator (R-) sectionxe2x80x9d in the SDH: see reference numeral 11A in FIG. 13) , is terminated and replaced at the LTE and the REG, and a line overhead [LOH (M-SOH)] 12 which, in communication between the LTE (referred to as xe2x80x9clinexe2x80x9d in the SONET, or xe2x80x9cmultiplex (M-)sectionxe2x80x9d in the SDH: see reference numeral 12A in FIG. 13), is terminated and replaced at the pieces of LTE.
Additionally, the overhead 10 is provided with the various operation and maintenance information. For example, as shown in FIG. 10, in the SOH 11 are defined A1, A2 bytes used to establish frame synchronization, a digital error supervising [BIP (Bit Interleaved Parity] byte B1 used on the section 11A, and data communication channel (DCC) bytes D1 to D3 (data link of 192 k b/s) for communication for a supervisory control in the section 11A. In the LOH 12 are defined a BIP byte B2 over a line 12A, APS (Automatic Protection Switch) bytes K1, K2, and DCC bytes D4 to D12 (data link of 576 k b/s) over the line 12A.
Moreover, in FIGS. 9 and 10, pointer bytes (AU [administrative unit] pointer) 13 shows, by using an address, a difference between a phase of a transmission frame and a frame phase of an administrative data unit (VT: Virtual Tributary Unit) contained in the payload 20. The pointer bytes 13 can rapidly establish frame synchronization of the VT.
Further, in the SONET, the basic transmission frame (STS-1) having the above-mentioned format is processed through byte multiplexing by n frames (where n=3, 12, 48, 192, and so forth), thereby forming an STS-n frame as shown in FIG. 11. It is possible to generate a high-speed signal with, for example, an STS-3 (of 155.52M b/s=51.84M b/sxc3x973) if the STS-1 frame is processed through the byte multiplexing by 3 frames, an STS-12 (of 622.08 M b/s) if processed through the byte multiplexing by 12 frames, anSTS-48 (of 2.488 G b/s) if processed through the byte multiplexing by 48 frames, and an STS-192 (of 9.953 G b/s) if processed through the byte multiplexing by 192 frames. Moreover, in the SDH, STM-N (N=n/3) respectively correspond to signals having the same transmission rates as those of the above STS-n.
Here, in the case of the STS-192, as shown in FIG. 12, the frame includes 9xc3x97576 (3xc3x97192) bytes of overhead 10 and 9xc3x9716704 (87xc3x97192) bytes of payload 20. However, all the bytes of the overhead 10 are not used. As shown in FIG. 12, only one byte is used for each area for operation and maintenance information (such as B1, E1, and F1) except special signals (such as A1, A2 bytes, and BIP byte B2). Hence, under the present circumstances, almost the entire area of the overhead 10 is left free.
(B) Description of SONET
FIG. 14 is a block diagram showing an illustrative SONET (transmission system). In the SONET 100 shown in FIG. 14, a 10 G ring network 200 for handling a transmission frame (STS-192) of about 10 G b/s, 2.4 G ring networks 300, 400 for handling a transmission frame (STS-48) of about 2.4 G b/s, and a 622 M ring network 500 for handling a transmission frame (STS-12) of about 622 M b/s are interconnected through transmission units serving as gateways which will respectively be described infra.
Further, as shown in FIG. 14, for example, in the 10 G ring network 200 (hereinafter briefly referred to as 10 G ring 200), a plurality of 10 G b/s line terminal multiplex relay transmission units (LTE) 110-1 to 110-4 and a plurality of 10 G b/s regenerator units (REG) 111 are connected in a ring manner. Similarly, in the 2.4 G ring networks 300, 400 (hereinafter briefly referred to as 2.4 G rings 300, 400), 2.4 G b/s LTE 120-1 to 120-4, and 130-1 to 130-4 are connected in the ring manner. In the 622M ring network 500 (hereinafter briefly referred to as 622M ring 500), 622M b/s LTE 140-1 to 140-4 are connected in the ring manner.
Moreover, in the 10 G ring network 200, according to a maximum transmittable distance of the line terminal multiplex relay transmission unit 110-i (where i=1 to 4), a proper number of regenerator units (REG) 111 are mounted between the line terminal multiplex relay transmission units 110-i to form the section 11A. It is to be noted that the regenerator unit 111 may be mounted in the 2.4 G rings 300, 400, or in the 622M ring 500.
Here, the transmission units (LTE) 110-i, 120-i, 130-i, and 140-i, and the transmission units 111 [hereinafter referred to as xe2x80x9ctransmission unit 100Axe2x80x9d or xe2x80x9cnode unit 100Axe2x80x9d unless the transmission units 110-i, 120-i, 130-i, 140-i, and 111 (LTE, REG) are individually shown] forming the rings 200 to 500 respectively have the function of performing replacement (termination/insertion) processing of the overhead 10 of the received transmission frame STS-n (STM-N). With attention to the function, as shown in FIG. 15, the transmission unit includes interface portions 171 to 173 according to a transmission frame to be handled (a speed of an accommodated line), a HUB circuit portion 174, a HED circuit portion 175, optical fibers 176, a CPU circuit portion 177, and so forth.
Further, each of the above interface portions 171 to 173 terminates a corresponding optical signal frame among, for example, an OC-192 (Optical Carrier-level 192) serving as an optical signal frame of the STS-192, an OC-48 serving as an optical signal frame of the STS-48, and an OC-12 serving as an optical signal frame of the STS-12, and extracts OH information in the overhead 10 to output the OH information to the HUB circuit portion 174, while inserting (storing) the OH information output from the HUB circuit portion 174 in the overhead 10 at a predetermined position.
However, the overhead 10 serving as a candidate for the termination/insertion processing varies depending upon whether it is processed in the interface portions 171 to 173 in the LTE 110-i, 120-i, 130-i, and 140-i, or in those in the REG 111. That is, both the SOH 11 and the LOH 12 are terminated in each of the LTE 110-i, 120-i, 130-i, and 140-i, and only the SOH 11 is terminated in each of the REG 111.
Moreover, all the interface portions 171 to 173 are not always used. For example, in the REG 111 shown in FIG. 14, the only type of interface portion, i.e., the interface portion 171 for the OC-192 is used.
Further, the HUB circuit portion 174 feeds the OH information from the interface portions 171 to 173 to the CPU circuit portion 177 through the HED circuit portion 175, while feeding to the corresponding interface portions 171 to 173 the OH information fed from the CPU circuit through the HED circuit portion 175. Here, the optical fibers 176 are used for connection with the HED circuit portion 175. It is thereby possible to perform high-speed OH information transmission processing between the HUB circuit portion 174 and the HED circuit portion 175 by using an ATM (Asynchronous Transfer Mode) cell-based optical signal.
Hence, as shown in FIG. 15, the HUB circuit portion 174 has a HUB portion 174A having the function of ATM cell generating (splitting)/multiplexing of the OH information from the interface portions 171 to 173, and an optical repeating regenerator (OR/OS: Optical Receiver/Optical Sender) 174B functioning as an electro/optical interface with the optical fibers 176.
Further, the above HED circuit portion 175 outputs to the CPU circuit portion 177 OH information input as an ATM cell through the optical fiber 176, while splitting into ATM cells and outputting to the HUB circuit portion 174 through the optical fiber 176 the OH information generated in the CPU circuit portion 175. For this purpose, the HED circuit portion includes an optical repeating regenerator (OS/OR) 175B functioning as an electro/optical interface with the optical fiber 176, and an overhead matrix portion 175B to make a routing control such that the ATM cell (OH information) from the HUB circuit portion 174 can be output to any one of a plurality of SCC (Serial Communication Channel) ports 178 of the CPU circuit portion 177. Moreover, the overhead matrix portion 175B also has the function of splitting into ATM cells the OH information generated in the CPU circuit portion 177.
Further, the CPU circuit portion 177 analyzes the OH information fed from the HED circuit portion 175, and generates OH information to be transmitted after being inserted in an overhead 10 of a transmission frame sent to an additional transmission unit 110, thereby making a supervisory control of the system 100. In the CPU circuit portion 177, the plurality of SCC ports 178 are connected to a CPU 180 through a bus 179.
According to the above configuration, in the above SONET transmission network 100, the termination/insertion processing of the overhead 10 is performed through the CPU 180 for each of the transmission units 100A, thus appropriately performing the transmission processing of the supervisory control information for the transmission network 100. This enables a network control unit 150 (see FIG. 14) to make a centralized supervisory control (such as construction of a communication path, or a network control (in operation) after the communication path construction) of the transmission network 100.
Specifically, the centralized supervisory control is typically made by communication between the CPUs 180 of the respective node units 100A by using the above D1 to D3 bytes or the D4 to D12 bytes for the DCC in the overhead 10.
Here, the communication path for communication is automatically constructed (set) by the network control unit 150 when, for example, the node unit 100A is turned on. That is, the network control unit 150 instructs, by using the DCC, one node unit 100A to set the communication path. The CPU 180 of the node unit 100A receives the instruction, and automatically and concurrently calls the CPUs 180 of all the other node units 100A by using the DCC [see FIG. 16(a)].
Further, the network control unit 150 determines a path (route) used for an actual communication (supervisory control) depending upon directions in which answers come back from the node units, and identification information (node ID) of the respective node units 10A, set to the answers [see FIG. 16(b)].
This enables the network control unit 150 to automatically set the communication (supervisory control) path irrespective of a network configuration. Moreover, when answers from the same node unit 100A come back from a plurality of directions, the network control unit 150 sets the path in a direction from which the first answer comes back.
Further, after setting the path as described above, the network control unit 150 communicates with the CPU 180 of a desired node unit 100A by using the DCC (through the set path), thereby making the centralized supervisory control of the transmission network 100 (for example, canceling an alarm generated in one node unit 100A). Moreover, after the communication path is set, the communication path may become unavailable due to, for example, a line trouble. In such a case, the network control unit 150 reconstructs a communication path as in the above discussion after the elapse of a desired time period (after, for example, about ten minutes).
However, in the system 100 using such a DCC, the centralized supervisory control requires a significantly complex protocol. Thus, it becomes difficult under the present circumstances to continuously obtain a stable operation. For example, when a large amount of alarms are given in one node unit 100A at the same time, the load on the CPU 180 of the node unit 100A rapidly increases, and a system down may be caused.
Further, when the communication path become unavailable due to a failure of the node unit 100A on the communication path in use, the line trouble, and so forth, the communication path can not be restored before, for example, the elapse of ten minutes as described above. Consequently, the restoration of the communication path takes a long time, resulting in a problem of reduced reliability of the centralized supervisory control.
Further, in the above system 100, a normal communication can be carried out with networks 160A, 160B (see FIG. 14) of other companies (other makers) since a recommendation provides a part concerning transmission of main signal, such as the frame synchronization establishment, parity operation, and handling of the payload 20. However, the companies have their own specifications for specifying another part concerning the communication between the CPUs 180, such as DCC. For example, in some cases, supervisory control information in the network 160A, 160B of the other companies can not directly pass through the 10 G ring 200.
Hence, in recent years, in the network (10 G ring 200) providing lines additionally connected to the network 160A, 160B of the other companies as shown in FIG. 14, it has been desired to perform no processing for supervisory control information other than own supervisory control information, and transparently pass the information to the other companies"" networks 160A, 160B (see FIG. 17).
In order to realize such a transparent transmission, there is one possible method in that a special byte for the purpose is additionally prepared (defined) in a space area (undefined byte) of the overhead 10, and the supervisory control information in the other companies"" networks 160A, 160B are set thereto (inserted therein).
However, the additional definition of the special byte causes a complicated control (handling) of the termination/insertion processing in the respective node units 100A, resulting in, for example, excessive load on hardware (mainly on the above interface portions 171 to 173), or an increase in hardware scale itself. Further, it becomes very difficult to control, for example, between which node unit 100A and which node unit 100A a certain byte should be transmitted.
In addition, in the system 100 of the same maker, though clock synchronization can basically be maintained at a time of termination/insertion of the overhead 10 in the node unit 100A, there may generally be left a slight difference. Consequently, little guarantee can be given that complete clock synchronization can be maintained when connected to the other companies"" networks 160A, 160B. Hence, when passing the supervisory control information received from the other companies"" networks 160A, 160B, data slip (loss or rereading) may occur in the node unit 100A due to a slight difference in clock.
That is, in the conventional system 100, the OH information of the overhead 10 is terminated in the respective node units 100A forming the section 11A or the line 12A, thereby continuously carrying out the communication about the supervisory control information through the CPUs 180 of the respective node units 100A. Hence, it is impossible to carry out a free communication for a supervisory control irrespective of the section 11A and the line 12A in the system 100. As a result, there are caused phenomena in that, for example, a stable supervisory control can not be made due to a rapid variation in load on the CPU 180, and OH information in conformance with specifications different from those of the own network can not be transmitted.
In view of the foregoing problems, it is an object of the present invention to provide an SDH transmission system, a frame transmission method in an SDH transmission system and an SDH transmission unit in which a desired communication can freely be carried out in a network irrespective of a relay section and a multiplexing section defined in an SDH transmission mode.
According to the present invention, for achieving the above-mentioned object, there is provided an SDH transmission system including a plurality of SDH transmission units for multiplex relay to carry out a multiplex relay transmission of a transmission frame in an SDH transmission mode, and at least one SDH transmission unit for relay to-carry out a relay transmission of the transmission frame between the SDH transmission units for multiplex relay, with a desired network formed by the plurality of SDH transmission units. Further, the SDH transmission system is provided with a relay section for a communication carried out by transmitting the transmission frame between the SDH transmission unit for multiplex relay and the SDH transmission unit for relay, or between the SDH transmission units for relay, a multiplexing section for a communication carried out by transmitting the transmission frame between the SDH transmission units for multiplex relay, and a network section for a desired communication carried out irrespective of the multiplexing section and the relay section in the network.
Therefore, it is possible to carry out a free communication in the network without much concern for the multiplexing section and the relay section, thereby providing the following advantages:
(1) It is possible to reduce the load of communication processing in the SDH transmission unit so as to stabilize the communication in the entire system.
(2) It is possible to make a normal connection through the network section to a network having specifications different from those of communication processing in the own network.
Here, in the SDH transmission system, an area for asynchronous communication may be reserved on an overhead portion of the above transmission frame, thereby carrying out the asynchronous communication in the above network section.
It is thereby possible to normally carry out the communication in the network without much concern for a clock of the transmission frame, resulting in a more simplified and higher-speed communication control in the network section.
Further, in the SDH transmission system, the area for asynchronous communication may be reserved on, in the above overhead portion, an overhead portion for a multiplexing section terminated in the SDH transmission unit for multiplex relay, thereby carrying out the asynchronous communication between the SDH transmission units for multiplex relay in the network section.
Further, in the SDH transmission system, the area for asynchronous communication may be reserved on, in the above overhead portion, at least an overhead portion for a relay section terminated in the SDH transmission unit for relay, thereby carrying out the asynchronous communication through the SDH transmission unit for multiplex relay and the SDH transmission unit for relay in the above network section.
It is thereby possible to carry out a communication with the desired SDH transmission unit for multiplex relay or relay in the network section, resulting in a contribution to flexibility of the communication in the network section.
Besides, in the SDH transmission system, an asynchronous communication cell having a communication control information portion and a data portion may be inserted in the above area for asynchronous communication, and the above SDH transmission unit may be provided with an asynchronous communication control portion capable of transmitting the asynchronous communication cell to the additional SDH transmission unit for multiplexing relay depending upon the communication control information portion of the asynchronous communication cell.
In the SDH transmission system, in the network section, the asynchronous communication cell is transmitted to the desired SDH transmission unit depending upon the communication control information portion thereof. Consequently, an extremely easy control can realize a high-speed communication in the network section. Additionally, it is possible to control a transmission destination (that is, a communication partner) of the above cell by simply controlling only the above communication control information portion, resulting in a very simplified control of the transmission destination.
Moreover, the communication in the network section may be made redundant. Thereby, it is possible to normally continue the communication in the network section even when the communication in the partial network section becomes unavailable due to occurrence of a failure or the like, resulting in significantly enhanced reliability of the system.
In this case, for example, two communication routes may be set in the above network section to carry out a communication by using, when one of the communication routes becomes unavailable, the other communication route. It is possible to extremely easily realize redundancy of the above network section.
Further, in the SDH transmission system, the above SDH transmission units may be connected in a ring manner to form a ring network as the above network, and communication routes respectively extending to the right and the left with respect to the above ring network may be set as the above two communication routes in the network section of the ring network. Thereby, it is also possible to extremely easily realize redundancy of the network section in the ring network.
Moreover, when any one of the above communication routes respectively extending to the right and the left is previously set to an unavailable state, only any one of the communication routes is used. Consequently, in the SDH transmission unit to terminate the communication routes, it is unnecessary to select the communication route, and continuously supervise in which of the communication routes the trouble occurs. Thus, it is possible to simplify, at least, the SDH transmission unit to terminate the communication routes.
Further, in this case, when the communication route different from the communication route set to the unavailable state becomes unavailable, the communication route set to the unavailable state may be set to an available state. In this case, it is also possible to normally continue the communication in the network section, resulting in significantly enhanced reliability of the system.
Besides, in the communication control information portion of the asynchronous communication cell may be set at least identification information of an SDH transmission unit serving as a transmission destination of the asynchronous communication cell. The above cell can thereby be transmitted to pass through intermediate SDH transmission units (a relay section, a multiplexing section) until the cell reaches the corresponding transmission unit in the network section. As a result, it is possible to extremely easily realize an asynchronous communication with a desired SDH transmission unit in the network section.
Moreover, in the communication control information portion of the asynchronous communication cell may be set both identification information of an SDH transmission unit serving as a transmission destination of the asynchronous communication cell, and identification information of an SDH transmission unit serving as a source of the asynchronous communication cell. In this case, the above SDH transmission unit serving as the transmission destination can identify that signals, even with the same transmission destination identification information, respectively contain different data as long as source identification information are different. As a result, even when signals with the same transmission destination identification information are transmitted from a plurality of transmission units, it is possible to continuously carry out a normal communication.
Besides, when two communication routes are set in the network section of the ring network, in the communication control information of the above asynchronous communication cell may be set identification information of the SDH transmission unit serving as a transmission destination of the asynchronous communication cell, identification information of the SDH transmission unit serving as a source of the asynchronous communication cell, and flag information used to identify through which of the respective communication routes the asynchronous communication cell is transmitted. Even when the respective communication routes are multiplexed in one SDH transmission unit, the communication route of the asynchronous communication cell can be identified depending upon the flag information. As a result, it is possible to surely perform selection of the communication route.
Further, when in the communication control information of a received asynchronous communication cell is set own identification information as the above identification information of the SDH transmission unit serving as the source, the above SDH transmission unit may discard the asynchronous communication cell. It is thereby possible to avoid a phenomenon in that, for example, a cell has no fixed destination by erroneously setting identification information which can not be found in the network section, and is left unerased indefinitely in the network, resulting in more enhanced reliability of the system.
Besides, as long as a received asynchronous communication cell can be transmitted to a network other than the above network, the above SDH transmission unit for multiplex relay can transmit the received asynchronous communication cell to the additional network as required. Thus, the above asynchronous communication can be applied to any type of network topology.
Next, in an SDH transmission system to transmit a transmission frame in an SDH transmission mode, a frame transmission method in the SDH transmission mode of the present invention includes the steps of inserting an asynchronous communication cell in a space area of an overhead portion of the transmission frame, and transmitting the transmission frame.
Thus, according to a frame transmission method in the SDH transmission mode of the present invention, it is possible to provide the following advantages:
(1) It is possible to make a supervisory control of the SDH transmission system by using the asynchronous communication, reduce the load of communication processing for the supervisory control, and make the above communication processing higher.
(2) Since the space area of the overhead portion is not fixedly used, it is possible to considerably enhance versatility and expandability of the communication processing for the above supervisory control.
(3) Since the asynchronous communication cell enables asynchronous transmission for the supervisory control without much concern for a clock of the transmission frame, resulting in an extremely easy communication control.
(4) It is possible to apply the existing asynchronous communication technique, resulting in very high practicability.
In addition, an SDH transmission unit of the present invention transmits a transmission frame in an SDH transmission mode. The SDH transmission unit includes a first overhead information extracting portion to extract first overhead information inserted in a space area of an overhead portion of a received transmission frame, a second overhead information extracting portion to extract second overhead information set in an area other than the above space area of the overhead portion, a main communication control portion to perform desired communication processing about the SDH transmission system depending upon the second overhead information extracted in the second overhead information extracting portion, an overhead inserting portion to insert a result of processing in the main communication control portion as an overhead portion of a transmission frame for an additional SDH transmission unit, and a distribution control portion to distribute to the overhead inserting portion at least first overhead information other than self-addressed information among the first overhead information extracted in the above first overhead information extracting portion so as to insert in a space area of the overhead portion for the additional SDH transmission unit.
Thus, according to the SDH transmission unit of the present invention, it is possible to provide the following advantages:
(1) Since the first overhead information other than the self-addressed information is transmitted after being inserted in the space area of the overhead portion of the transmission frame for the additional SDH transmission unit, it is possible to reduce the load of communication processing for the supervisory control.
(2) Since the space area of the overhead portion is not fixedly used, it is possible to considerably enhance versatility and expandability of the communication processing for the above supervisory control.
Here, the above first overhead information extracting portion may be configured as an asynchronous communication cell extracting portion to extract an asynchronous communication cell having a communication control information portion and a communication data portion, and the above distribution control portion may be configured as an asynchronous communication control portion to perform the above distribution processing depending upon communication control information set in the communication control information portion of the above asynchronous communication cell.
In this case, according to the SDH transmission unit, it is possible to provide the following advantages:
(1) The above communication processing can be made higher.
(2) Since the asynchronous communication cell enables asynchronous transmission for the supervisory control without much concern for a clock of the transmission frame, resulting in an extremely easy communication control.
(3) It is possible to apply the existing asynchronous communication technique, resulting in very high practicability.
Here, the above asynchronous communication control portion may include a switching mechanism portion to output, when communication control information of the received asynchronous communication cell is self-addressed information, the asynchronous communication cell to the above main communication processing portion, while outputting, when the communication control information of the received asynchronous communication cell is information other than the self-addressed information, the asynchronous communication cell to the above overhead inserting portion. It is thereby possible to realize the above distribution function in a simplified configuration.
Further, the above asynchronous communication control portion may include an asynchronous communication cell generating portion to generate an asynchronous communication cell addressed to an additional SDH transmission unit. In this case, the above switching mechanism portion may be set to output to the above overhead inserting portion the asynchronous communication cell generated in the asynchronous communication cell generating portion so as to insert in a space area of the overhead portion for the additional SDH transmission unit. It is thereby possible to transmit with a simple setting at a high speed a cell which needs to be posted to the additional SDH transmission unit.
Besides, the above asynchronous communication cell generating portion may set in the communication control information portion of the above asynchronous communication cell identification information of the additional SDH transmission unit serving as a transmission destination of the asynchronous communication cell. The above cell can thereby be transmitted to pass through intermediate SDH transmission units until the cell reaches the corresponding transmission unit. As a result, it is possible to extremely easily realize an asynchronous communication with the SDH transmission unit.
Further, the asynchronous communication cell generating portion may set in the communication control information portion of the above asynchronous communication cell both identification information of the additional SDH transmission unit serving as a transmission destination of the asynchronous communication cell, and own identification information. The SDH transmission unit serving as the transmission destination can identify that signals (cells), even with the same transmission destination identification information, respectively contain different data as long as source identification information are different. As a result, even when cells with the same transmission destination identification information are transmitted from a plurality of transmission units, it is possible to continuously carry out a normal communication.
Further, the above switching mechanism portion may discard, when own identification information is set in a communication control information portion of an asynchronous communication cell received from an additional SDH transmission unit, the asynchronous communication cell. It is thereby possible to avoid a phenomenon in that, for example, a cell has no fixed destination by erroneously setting identification information of an SDH transmission unit which does not exist, and is left unerased indefinitely in the network, thus greatly contributing to enhancement of reliability of the SDH system.
In addition, the switching mechanism portion may transmit, when in the communication control information portion of the above asynchronous communication cell is set transmission instruction information showing as a destination an SDH transmission unit in an additional SDH transmission system, the asynchronous communication cell to the SDH transmission unit in the additional SDH transmission system. Since it is thereby possible to transmit the received asynchronous communication cell to the additional network as required, the asynchronous communication can flexibly be applied to any type of network topology.